Glass seal manufacture



Och 1969 e. F. STOCKDALE ETAL 3,472,413

GLASS SEAL MANUFACTURE 4 Sheets-Sheet 1 Filed Sept. 8, 1966 J'nVenfm-s A'J'rmwz: 2' Mu/va Al. Mar:

lye/2i United States Patent M 3,472,413 GLASS SEAL MANUFACTURE George F. Stockdale, Trenton, and Edmund N. Metz, Pennington, NJ., assignors to RCA Corporation, a corporation of Delaware Filed Sept. 8, 1%6, Ser. No. 577,863 Int. Cl. C03c 23/20, 27/04; H01j 9/26 US. Cl. 2202.1 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a new and improved method of sealing a glass window to the periphery of a slot in the envelope of acathode ray tube to be used for contact printing.

A contact printing cathode ray tube usually comprises a metal or glass envelope including an enlarged end portion or faceplate on which a narrow, elongated, aluminized phosphor screen is formed, an opposite end portion containing an electron gun for projecting an electron beam toward the screen, and means for scanning and modulating the beam over the surface of the screen in accordance with an input signal. In the case of an opaque metal envelope, a long narrow slot is formed in the faceplate of the envolope, a transparent thin window, e.g. of glass or mica, is sealed to the envelope over the slot, and the inner portion of the window over the slot is coated with phosphor and aluminized to form the screen. In the case of a glass envelope, although the envelope is transparent, the envelope cannot be made with a single thickness which is great enough to withstand atmospheric pressure and also thin enough to prevent excessive parallax through the screen portion. Thus, the faceplate of the glass envelope is also formed with a long narrow slot to which a thin glass or mica window is sealed and screened. In operation of the tube a photosensitive medium may be moved over the face of the tube and the phosphor screen of the tube excited to thereby photographically record the desired information on the medium;

In the past, windows have been scaled to metal or glass envelopes by sandwiching a layer of low-softening-point glass frit between the window and the envelope and firing the assembly in an oven at a temperature above the softening temperature of the frit. For glass envelopes,'the frits selected must be softenough to fire completely under conditions of temperature and time that will not cause the glass to sag. These conditions severly limit the selection of frits that can be used with many base glasses. Moreover, when a glassseal is made in a cathode ray or other electron tube with common low-softening-point frits it is not possible to bake out the tube during subsequent processing at a temperature above the softening temperature of the frit without damaging the seal. I

An object of the present invention is to provide a new and improved method. of sealing a glass window to .the slotted faceplate of a cathode ray tube. I I

In one practice of the method, a thin glass window may be sealed to a slotted faceplate of metal or glass by positioning a thin sheet of glass over the faceplate slot, supporting the portion of the sheet above the slot to prevent 3,472,413 Patented Get. 14, 1969 sagging into the slot during the sealing operation, and sealing the sheet to the faceplate by a progressive radiant heat method. This method involves subjecting a narrow elongated region only of the sheet at a time to radiant heat suflicient to heat the glass in that region to fusing temperature, causing the heated region to move slowly over the surface of the sheet, to progressively seal the sheet to the faceplate around the slot, and then slowly cooling the assembly to room temperature.

Instead of sealing the glass sheet directly to the faceplate, the faceplate is preferably first coated with a layer of glass frit, and the layer is fired before sealing the window thereto. I

Preferably, the portion of the window overlying the faceplate slot is supported in a position slightly above the edges of the slot when the parts are hot to cause the glass sheet to sag differentially by gravity at the slot edges during the sealing operation and thereby give that portion of the window an outwardly convex shape after cooling. In the examples given, the faceplate is curved convex outwardly and a flat sheet of glass is sagged onto the curved faceplate.

In the accompanying drawing,

FIG. 1 is an end view of the funnel portion of a cathode ray tube made according to the present invention;

FIG. 2 is a longitudinal section View taken on the line 2-2 of FIG. 1;

FIG. 3 is a transverse section view taken on the line 33 of FIG. 2;

FIGS. 4 and 5 are enlarged fragmentary views, similar to FIG. 3, of modifications thereof;

FIG. 6 is a longitudinal section view of an apparatus for sealing a glass window to the faceplate of a cathode ray tube in accordance with the invention;

FIG. 7 is a transverse section view taken on the line 7-7 of FIG. 6; and

FIG. 8 is a graph of viscosity plotted against temperature for two glass compositions.

Referring to the drawing in detail, FIGS. 1-3 show a metal funnel portion 1 of a cathode ray tube suitable for use in contact printing apparatus. This funnel portion 1 comprises a flange 3 for connection to the gun portion (not shown) of the tube, an outwardly flared portion 5, and an elongated rectangular faceplate 7. The faceplate 7, which is preferably curved outwardly as shown in FIG. 3, is formed with a narrow elongated slot 9. The slot 9 is closed by a thin transparent glass window 11 which is sealed to the faceplate 7 around the slot by a novel method to be described later. Subsequent to the sealing of the window to the faceplate, the portion of the inner face of the window exposed through the slot is coated with phosphor and aluminized to form an elongated fluorescent viewing screen 13 for excitation by an electron beam in operation of the tube. As used in contact printing apparatus, letters or characters imaged on the: screen 13 excite a sensitive film or tape 15 adjacent to the window 11, as shown by the dashed lines in FIG. 3. For this purpose, the central portion 11' of the window 11 is preferably curved outwardly or convex as shown.

The thin glass window 11 may be sealed directly to the metal faceplate 7 as shown in FIG. 3. However, best results are obtained by providing an intermediate vitreous layer 17 of a glass frit having softening and annealing points (temperatures) somewhat lower than the glass of the Window 11, as shown in FIG. 4. In any case, the faceplate 7, Window lland frit layer 17 (if used) should be compatible, that is, having matching coefficients of thermal expansion. For example, if the faceplate 7 is made of Kovar, which has a thermal coefficient of 45.4 to 50.8 X 10 C. (30400 C.), the window 11 may be of Kimble glass EN-l, which has a softening point of about 710 C., an annealing point of about 480 C. and a ther- 3 mal coefiicient of about 47 10-' C. (O300 C.), and the glass frit may be of Kimble SG7 Vitreous Solder Glass, which has a softening point of about 572 C., an annealing point of about 476 C. and a thermal coeflicient of about 46X 10 C. (300* C.). Both of these glass compositions are compatible with Kovar.

FIG. 5 shows an alternative to FIG. 4 wherein the funnel portion 1' is made of relatively thick glass, such as Kimble tEN-l, and the thin window 11 and frit layer 17 are sealed to the glass faceplate 7', by the method of the present invention.

As an example, the method of sealing the window 11 to the envelope faceplate 7 of FIG. 4 will be explained in detail in connection with FIGS. 6 and 7, with the understanding that the invention is not limited to all of the specific details described.

First, the outer face of a Kovar faceplate 7 is prepared by cleaning in a vapor degreaser and hot deionized water, drying, firing in wet hydrogen at about 1050 C. for about 30 minutes to anneal and de-carburize the surface, and oxidizing by a flame. Then, the adhesive side of a Vitta G-l005 glass transfer tape 17, made of Kimble SG7 solder glass plus organic binders, is pressed apainst the outer face of the faceplate 7 over the slot 9 (see FIG. 4). The clear plastic backing is removed from the tape 17 and the tape is trimmed from the slot 9 with a razor blade.

Next, the envelope 1 (with the frit layer 17 on the faceplate 7) is mounted within the combined oven and radiant heating apparatus shown in FIGS. 6 and 7. This apparatus comprises a box-like housing 19, made up of four side walls 20-23 and a top Wall 24 of firebricks, mounted for sliding movement on the bed of a lathe schematically shown by the base plate 25. The envelope 1 is fixedly mounted on the base plate 25 by one or more blocks 27 and clamps 29 which engage the flange 3. An electric heating coil 31 is mounted along the inner wall of each of the two long sides 22 and 23 of housing 19 to heat the interior thereof, as an oven. In addition to the coils 31, a diameter resistive rod 33 of silicon carbide, for example, extends through holes in the walls 22 and 23 and across the housing 19 at a distance of about from the slot 9 of the envelope, for locally heating only a narrow region of the frit layer 17 and window 11 at a time by radiant heating.

The glass frit layer 17 on the faceplate 7 is fired or glazed in the housing 19 by first heating the coils 31 to produce an oven temperature suflicient to remove the organic binders from the frit layer 17, e.g. 480 C., then heating the rod 33, by passing a suflicient current therethrough, to about 1200-l400 C., and slowly moving the rod 33 (by moving the housing 19) from one end of the frit layer 17 to the other at a speed such that the temperature of the frit layer 17, produced by the radiant heat from rod 33, is about 650 C., which is above the softening point of the frit. As the rod 33 moves slowly along the frit layer 17, only a narrow region of the layer is fused beneath the rod at a time, which region moves with the rod along the layer 17. Where the envelope is of glass, instead of metal, only a small region of the envelope is softened at one time, and hence, the envelope shape is maintained.

The next step is to seal the window 11 to the fired layer 17. After firing the frit layer 17, as described above, the envelope 1 is removed from the housing 19 and one or more flat support strips 35 of a material such as graphite or boron nitride, which will not stick to hot glass, are adjustably mounted in the slot 9 by means of a jig 37, made up of a bar 39 having a groove 41 in which the strips 35 are positioned, and a block 43 adjustably mounted on the block 27 by screws 44. The strips 35 should fit loosely in the slot 9 and have rounded upper surfaces. The height of the support strips 35 is adjusted so that when the parts are hot the strips project slightly above the sides of the slot 9, to produce the desired curved portion 11' of the window 11. For example, this projection may be about mils for a slot 100 mils wide.

The envelope 1, with the vsupport strips 35 and jig 37 in place, is mounted in the housing 19 and a thin flat rectangular strip 11 of glass is laid on the strips 35, as shown in FIGS. 6 and 7. The strip may be a 5-7 mils thick sheet of Kimble EN1 glass. The oven is heated slowly to heat the glass strip 11 to its annealing point (480 C.), and then the rod 33 is heated to about 1350 C., at which temperature the adjacent narrow portion of the strip 11 is heated to about 675 C., i.e. slightly below its softening point (710 'C.). Starting'with the rod 33 at the extreme end of the strip 11 (the right end in FIG. 6) the localized radiant heat from rod 33 softens the strip 11, causing it to sag by gravity onto the curved surface of the faceplate 7 and its fired frit layer 17,,and to fuse to the layer 17. The heated rod 33 is moved slowly, at about 1 per minute, along the length of the window strip 11, to sag and seal progressively the entire window 11 to the faceplate 7 around the slot 9. As in firing the frit layer, only a narrow region of the glass is fused beneath the rod 33 at a time, which region moves with the rod. When the seal is completed, the current to the rod 33 is umed off, and the assembly is allowed to cool slowly through the annealing zone to roomtemperature, to anneal the glass window.

The same method, with the same tempraturs, may be used to seal the thin glass window 11 over the slot 9' in the thick walled glass envelope 1' shown in FIG. 5, if Kimble EN-l glass is used for the envelope. In this case, an additional advantage of the local radiant heating is that the window can be sealed to the envelope without deformation ofthe envelope even when the same kind of glass is used for the window and the envelope. .If the envelope 1 is made of a glass having a higher softening point than the window 11, the oven temperature should be correspondingly higher.

FIG. 8 shows the viscosity-temperature curve A for the Kimble SG7 solder glass frit and curve B for the Kimble EN1 glass. As shown, these two glasses have substantially the same strain point, at about 456 C., but quite different softening points.

In using the present process for sealing a glass sheet or window directly to a glass or metal envelope (without the intermediate frit layer 17), the outer urface of the faceplate is cleaned and the envelope section 1 or 1', support plates 35 and flat glass Window llare assembled and placed in the housing 19 of FIGS. 6 and 7. If the envelope is of Kovar (metal), any glass that is compatible with Kovar may be used for the window 11, as for example, Kimble EN-l glass. If the envelope is of glass, the window 11 may be the same glass as the envelope (if the wall thickness of the envelope is large compared to the window), or a different .glass having a lower softening point than the window glass. With either glass or metal for the envelope, the process of progressively sagging the flat glassstrip 11 onto the faceplate and sealing the strip 11 around the slot 9 to form the window is. similar to that described above for the-window-frit seal, except for the temperatures. of the radiant heating rod 33 and local region of the glass strip. Without the frit layer, it is necessary to heat the rod 33 to a considerably higher temperature, sufficient to produce a fusing temperature of 1000-1100 C. in the narrow region of the glass beneath the rod, in order to seal the glass to the faceplate 7. In the case of a glass envelope having a softening point substantially lower than this fusing temperature, it may be necessary to cool the opposite (inner) surface of the faceplate 7 to prevent collapse thereof during the sealing operation.

The method disclosed herein for progressively firing the frit layer 17 and progressively sealing the glass window 11 to the fired frit layer 17, excluding the step of supporting the portion of the window over the slot 9 during the sealing operation, is claimed in a copending application of George F. Stockdale, Ser. No. 577,864, filed concurrently herewith.

What is claimed is:

1. In the manufacture of a cathode ray tube having a faceplate with a narrow elongated slot closed by a thin glass Window; the method of sealing said Window to said faceplate, comprising the steps of:

(a) cleaning the outer surface of said faceplate surrounding said slot;

(b) placing an elongated thin strip of glass over said surface and said slot;

(0) supporting the portion of said glass strip overlying said slot to prevent sagging of said portion by gravity into said slot during the sealing operation;

(d) locally heating only a transverse narrow elongated region of said strip at a time by radiant heat to raise the temperature of the glass in said region to a temperature sufiicient to fuse the glass in said region to said surface; and

(e) slowly moving said heated region along the length of said strip to progressively fuse and seal the strip to said faceplate.

2. The method of claim 1, wherein said portion is supported in step (c) in a position slightly above the edges of said slot when the parts are hot, to cause said strip to sag differentially by gravity at the edges of the slot during the sealing operation and thereby give said portion a convex shape after cooling.

3. The method of claim 1, wherein said faceplate is outwardly convex, and a flat strip of glass is sagged by gravity onto said convex faceplate in steps (d) and (e).

4. The method of claim 1, wherein said faceplate is made of a metal having a melting point higher than said fusing temperature.

5. The method of claim 1, wherein said faceplate is made of a glass having a softening point at least equal to that of said glass strip, and the assembly is heated to the annealing point of the faceplate prior to step (d) and cooled slowly through the annealing range after step (e).

6. In the manufacture of a cathode ray tube having a faceplate with a narrow elongated slot closed by a thin glass window; the method of sealing said window to said faceplate, comprising the steps of:

(a) cleaning the outer surface of said faceplate surrounding said slot;

(b) applying to said cleaned surface a layer of glass frit;

(c) firing said layer;

(d) placing an elongated thin strip of glass having a softening point higher than the softening point of said glass frit over said fired surface and said slot;

(e) supporting the portion of said glass strip overlying said slot to prevent sagging of said portion by gravity into said slot during the sealing operation;

(f) slowly heating only a transverse narrow elongated region of said strip at a time by radiant heat to raise the temperature of the glass in said region to a fusing temperature between the softening points of said frit and said strip to fuse the glass in said region to said frit layer; and

(g) slowly moving said heated region along the length of said strip to progressively fuse and seal the strip to said faceplate and thereby form a thin transparent window over said slot.

7. The method of claim 6, wherein said portion is supported in step (e) in a position slightly above the edges of said slot when the parts are hot, to cause said strip to sag differentially at the edges of the slot during the sealing operation and thereby give said portion a convex shape after cooling.

8. A cathode ray tube having a face-plate window sealed by the method of claim 7.

References Cited UNITED STATES PATENTS 2,598,286 5/ 1952 Mulder et al 40 3,120,433 2/1964 Van Zee 6543 FOREIGN PATENTS 964,709 5/ 1957 Germany.

S. LEON BASHORE, Primary Examiner S. R. FRIEDMAN, Assistant Examiner US. 01. X.R. 65-43, 59, 107, 154, 15s; 220-2. 3 

